1. Field of the Invention
The present invention relates to a magnetic recording or reproducing apparatus which employs a magnetic sheet or a magnetic tape, etc. as a recording medium and makes a recording or reproduction by using a magnetic head.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a recording-reproduction change over circuit for a magnetic head in a conventional magnetic recording or reproducing apparatus, wherein 1 is an input terminal into which a recording signal is fed, and 2 is a DC power source terminal made to have DC potential Vcc and to be connected to a power source only when a recording is made. 3 is a switch to render the impedance between a point A and the ground to "0" only when a reproduction is made and 4 is a switch to render the impedance between a point B and the ground to "0" only when a recording is made. 5 is a magnetic head, 6 is an amplifier for reproduction to amplify a reproducing signal from the magnetic head, and 7 is an output terminal, wherein an amplified reproducing signal is delivered at this output terminal 7 and then is fed into a reproduction processing circuit. Q1, Q2 are transistors, C1, C2, C3, C4 are capacitors, and R1, R2, R3, R4, R5, R6, R7, R8 are resistors, wherein R7 is a resistor for setting a frequency characteristic of the recording signal, and R8 is a resistor for setting a frequency characteristic of the reproducing signal. The resistors R7, R8 are not indispensable and either one of them suffice. And it is an ordinary practice for reducing a distortion in the recording current to provide the relationship of the resistance values to be R1=R4, R2=R3, R5=R6.
The switches 3, 4 are ordinarily made of transistors and are to render the impedance between, respectively, the point A, point B and the ground to virtually "0".
A conventional recording-reproduction change over circuit has the above-mentioned arrangement, and the recording signal fed into the input terminal 1, when a recording is made, is amplified by the transistors Q1, Q2 with an amplification factor of 2.times.L.sub.H / R5, an then is delivered at the point A. Since the impedance at the point B is "0" when a recording is made, if the level of the recording signal is expressed by V.sub.R, a recording current of V.sub.R .times.(2/R5) will flow to the magnetic head 5 and will be recorded on the magnetic medium. Here, L.sub.H is an inductance of the magnetic head 5.
Next, when a reproduction is made, the impedance at the point "A" is "0" by the switch 3. The signal recorded on the magnetic recording medium will be reproduced by the magnetic head 5 and will be amplified by the amplifier 6 for reproduction. The amplified signal will be delivered at the output terminal 7 and will be fed to a reproduction processing circuit.
Now, the charging of the capacitor C3 is made in such manner that the DC potential at the point A when a recording is made will be Vcc/2 and the potential difference between the both ends of the capacitor C3 for eliminating the direct current component of the recording signal will be Vcc/2.
If a recording mode is changed over to a reproduction mode under this state the potential at the point A becomes "0" potential, and discharging current of the capacitor C3 flows to the magnetic head 5 as shown in FIG. 1. This signal will be recorded on the magnetic recording medium and the information previously recorded on the magnetic recording medium will be damaged thereby while this discharging current is flowing to the magnetic head 5.